Resonant plasma x-ray source

ABSTRACT

An improved efficiency x-ray source comprised of a fluorescent x-ray tube and resonant high voltage power supply. The fluorescent x-ray tube is an arc discharge tube filled with a low-pressure vapor, xenon for example, that is excited by high-frequency, high-voltage pulses to produce x-rays. The power supply passes arcs through the tube that produce significantly more radiation per unit energy than equivalent conventional vacuum x-ray tubes. The power supply may be a high frequency resonant AC supply or it may be rectified to give resonant DC. The fluorescent tube is driven in cold cathode mode, avoiding a fragile filament. The arc gap may also be large or very small in order to serve as a broad beam source or point source.

TECHNICAL FIELD

The present invention relates to x-ray tube design and x-ray tube powersupply design. More particularly, the present invention relates to thedevelopment of a high efficiency x-ray source consisting of afluorescent x-ray tube, and resonant power supply, which relies onplasma within the tube. The present invention further relates to thedesign of a power supply to achieve enhanced efficiency. This x-ray tubedesign can then be used in applications such as product irradiation, andmore particularly the sterilization of materials such as foodstuffs andmedical implements.

BACKGROUND OF THE INVENTION

As public demand for greater safety from potentially harmfulmicroorganisms increases, scientists must come up with more effectiveand efficient ways of providing safe products and environments. Onetechnique that is well suited to the reduction in the quantities ofmicroorganisms and pests is irradiation.

Irradiation uses relatively high doses of one of several forms ofradiation, gamma rays, electron beam (e-beam), or x-rays, to killmicroorganisms and pests that may be present in or on a given material.The radiation ionizes atoms that are sometimes part of criticalmolecules such as DNA and RNA. Damaging key cell components such asthese causes the cells to die, and if enough cells are killed, then theorganism dies. There are two main forms of irradiation in use today.They are gamma irradiation and e-beam irradiation. Gamma irradiationuses a radioisotope source such as cobalt-60 that emits gamma raysmeasured in the millions of electron volts (MeV), while e-beam uses anaccelerator to accelerate electrons to MeV range energies. Although bothtechnologies have performed well in limited situations, significantimprovements are required to make this technology more accessible.

Gamma irradiation has the major drawback of using radioisotope sources.Radioisotopes cannot be turned off and therefore create a disposalhazard. Additionally, there is public perception linking allradioisotopes to atomic bombs and various accidental radiation deaths,as well as fear that the object being irradiated will be contaminated orsomehow become radioactive, even if it cannot. All this makes itdifficult to sell the public on the benefits on gamma irradiation. Thehigh energy MeV range gamma rays also require a significant amount ofshielding, leading to the irradiation facilities being very large,usually requiring their own building with elaborate shielding andconvoluted conveyor systems to safely move the product through the highradiation area. It should be noted also that the gamma rays mostly gothrough the material without loosing much energy, i.e., without creatingmuch ionization. On the positive side, irradiation sources areinexpensive, stable and require no power to produce the radiation. Butwhile the source itself is inexpensive, the irradiation facility itselfis expensive—often costing a million dollars or more. Further, due tothe nature of the shielding requirements for radioisotopes, the use ofgamma irradiation usually requires a completely separate facility fromthe manufacturer or distributor and thus results in additional expensesassociated with shipping, loading, and packing the materials beingirradiated. On top of all this, add the burden of meeting US NuclearRegulatory Commission and associated state regulatory bodies rules forhandling radioactive material.

E-Beam irradiation has several major drawbacks as well. The acceleratorsare expensive (usually in the million to several million-dollar range)and are fairly big requiring a large room or separate building. Further,unlike gamma irradiation that can penetrate through fairly thickmaterials (even metals), electrons only travel a short distance in mostproducts. For example, a typical e-beam may only penetrate ¼ inch (6 mm)in hamburger meat, and is only effective near the surface of materialscomposed of heavier atoms such as steel. This lack of penetration depthdoes lead to a benefit in that it may require less shielding if there isnot much secondary x-ray production, but the limitations prevent thetechnology from being useful in many cases. E-beam technology is alsousually part of a separate facility as well, creating the same types oftransport problems as gamma facilities. Similarly, accelerators must belicensed with the states and are carefully controlled as one of the moredangerous electronic radiation producing products available.

It is also possible to have electrons from an accelerator shine on aheavy metallic target to produce high-energy x-rays or low-energy gammarays that can in turn be used much in the same way as gamma irradiationfrom radioisotopes. Unfortunately, the percentage of e-beam energyconverted into x-rays energy is only about 1 percent and the overallefficiency is much less than that. Thus, an e-beam x-ray system could beconsidered the worst of both worlds in that now heavier shielding isrequired with a much more expensive and inefficient source. A full-scalecommercial irradiation facility built on this principle would prettymuch require its own separate power plant. With the source being soinefficient that the technique is not economically viable except as anoccasionally used add-on feature to an otherwise useful e-beam system.

Therefore, in light of all these problems, a need exists for a devicethat: (1) is small enough to be integrated into the sites where they areneeded; (2) achieves an optimal penetration depth for the product beingtreated; (3) is safe enough for use by an average person; (4) usesavailable power efficiently, and (5) is low in cost.

Low energy x-rays appear to meet most of these requirements since theycan be tuned so that a maximum amount of x-ray energy is absorbed in agiven product. X-ray tubes and power supplies are small and inexpensiveand can be made in a wide variety of sizes. Television sets are oneexample of small economical x-ray producing device since they containthe high voltage supply, vacuum tube and other components that arenecessary at very low cost, but use shielding to minimize x-rayemissions.

A traditional x-ray tube is made of a glass or ceramic envelope and isevacuated to a high vacuum. The envelope usually has an x-raytransparent window, typically made of beryllium, aluminum, or glass. Thex-ray tube may have x-ray shielding, cooling, and high voltageinsulation incorporated into its design as well. The tube has a filamentat one end that is intensely heated so that it easily supplies electronswhen a high voltage potential is applied between it and the anode. Theanode is typically a large block of metal that normally is copper (dueto its heat conduction), with a different target material often brazedto the surface that the electrons strike. The vacuum x-ray tube requirestwo power supplies: a DC power supply for the filament heating whichtypically operates at low voltage (0-10 volts typical) and a few wattsof power; and a second power supply that provides a high voltage (5-200+kV) DC supply that may range in power from a few watts to 100 kilowattsor more.

Traditional x-ray tubes, however, still suffer from a number of knownproblems associated with efficiency. When electrons hit the targetmaterial of the x-ray tube, they loose the energy they gained from beingaccelerated by the high voltage electrical potential existing betweenthe filament and the target anode. Through scattering and ionization,the electrons lose energy by transferring some of it to the atoms in theanode target material. For each scattering and ionization event, x-raysand lower energy light are emitted, creating a spectrum of energy thatis made up of a continuum of x-rays given up through scattering, andcharacteristic x-rays of the target material. The efficiency of theconversion of electrical energy to x-ray energy is sometimes expressedby a simple empirically derived formula of the form E_(x)=E*kZV^(x)where E_(x) is the x-ray energy, E is the electrical energy, k is aconstant, Z is the atomic number of the target, V is the voltage, and xis a power generally accepted to be a little less than 2. By using ahigher atomic number target material or higher voltage, it is possibleto raise efficiency. Tungsten is a very popular target material for thisreason, along with its high melting point and reasonably good thermalconductivity. Other heavy atoms have too low a melting point to beoptimal in high-energy x-ray tubes. A tungsten target tube operated at50 kV potential is approximately 0.7% efficient at converting the energygoing into the tube to x-ray energy. When one includes the power supplyefficiency, the overall energy efficiency for generating x-rays is lessthan 0.5%, and then the x-ray beam is further reduced by the windowdiameter or by collimators that typically allow less than one percent ofthe x-ray flux to be utilized. This combination of factors results in aneffective use of the energy applied to the x-ray tube of less than 50parts per million (0.005%). The result of these inefficiencies is x-raytubes and power supplies that are very large and expensive and nearlyall of the energy applied becomes waste heat. A small cabinet systemthat holds less than a cubic foot of material would require a 500-kVAtransformer, which is a typical size transformer for an entire smallbusiness. Ultimately this wasteful use of energy limits who canpractically own and operate x-ray systems for vital uses such as inmedical imaging equipment, and makes x-ray tubes unfeasible for certainnew applications such as the sterilization of food, medical utensils andproducts, and countless other beneficial applications of x-rays.

In addition, traditional x-ray tubes, as is also the case with commonlight bulbs, suffer from frequent filament failure. In both x-ray tubesand light bulbs, the filament is usually tungsten or a tungsten alloy.Over time the tungsten is vaporized, weak spots form, and eventually itbreaks. Much of the design improvements over the past 100 years havebeen directed toward ways of improving filament life through bettermaterials, better cleanliness, and the use of higher vacuum. Whilefilament life has improved, tube life times are typically in thehundreds of hours when operated at anywhere near their peak voltage andcurrent specifications. A side affect of the improvements has been todramatically increase the manufacturing cost.

The traditional x-ray design has also been driven mostly by the x-rayimaging industry, either medical or industrial, leading designers todevelop x-ray tubes with very small focal spots on the anode where theelectron beam strikes. While this is a very desirable trait for imaging,it is not desirable when a broad beam source is needed for suchapplications as sterilization of materials, food irradiation, or x-rayfluorescence. The standard x-ray tube design is inherently a pointsource design and broader beams are achieved by using larger sidewindows or end window tube designs that have tighter anode to windowgeometries allowing for a wider angle exit path. The tube still must bemoved farther away from the target being irradiated in order to coverlarger areas. The incident dose rates drop with the square of thedistance from the source, making the traditional designs even lessefficient when a broad beam is required.

It has been known for much of this century that a lamp filled withlow-pressure vapors will give off x-rays when a high voltage is appliedacross it, and during the past few decades there has been a lot ofexperimental and developmental work on flash x-ray or plasma pinch x-raydevices. They produce x-rays through scattering and electron excitationof the vapor and electrodes as well as the plasma pinch effect thatoccurs when the magnetic field created by the arc collapses. Flash x-raydevices consist of an x-ray tube filled with a low-pressure vapor and ahigh voltage capacitive discharge power source. Flash x-ray tubes aregenerally used for taking high-speed x-ray radiographic images inapplications such as ballistics. Their power supply topology limits boththeir frequency and power, limiting their usefulness as a general sourceof x-rays. Plasma pinch devices, of which the flash x-ray tube is thesimplest version, are being studied intensively as a means ofcompressing nuclear fuel for fusion. Several very high power deviceshave been produced but the design of their power supplies have stilllimited them to pulse operation mostly due to the design goal ofigniting a plasma with a single pulse and then maintaining it withoutadditional pulses. To date, the power supplies for these devices consistof a high voltage DC power supply that charges high voltage capacitors,and has a switching mechanism to discharge the capacitors through thetube. The pulse can be as short as tens of nanoseconds to severalmicroseconds in duration. The recharge and cycle rates of the capacitivedischarge systems are very slow, typically less than ten per second.Faster types can be made, but are usually lower in power. Both the speedand total power limitations are inherent to the charge-discharge cycleof capacitors. This makes flash x-ray unsuitable for medium and highpower continuous operation.

What is important about flash x-ray devices and their cousins, laserablation x-ray sources, is that both have been shown experimentally tohave efficiencies that are, when designed properly, four times higherthan a traditional x-ray tube, possibly more. Therefore, a need existsfor a new way of driving the flash x-ray device that would allow forhigh continuous power output at high efficiency to meet the needs of theirradiation application. Much in the same way that the world isconverting to fluorescent lighting because it is inherently moreefficient than tungsten lighting, a need exists for a fluorescent x-raysystem.

Although fluorescent x-ray tubes and power supplies have not beencommercially developed for purposes of irradiation, some of theprinciples underlying the present invention have been used in flashlamps and neon lights. A flash lamp is usually designed to emit a brightflash of light or operated at a higher pulse frequency whereby it canlook like it is on constantly to the human eye. A neon light operates atline frequency (60 Hz in the US) or with some newer supplies at 20 kHzor more. Either tube is made of glass or quartz and has two electrodes,which are commonly made of tungsten, predominantly for its high meltingpoint and thermal conductivity. The tube is filled with a vapor that maybe at several times atmospheric pressure (1 atm.=760 torr) to 20 torr orless. In order to produce free electrons, a high voltage trigger pulseis usually used to ionize the gas. Then it is operated at lower voltageto produce light. With a large amount of vapor present, the vaporbecomes very conductive and effectively shorts out as an arc ofelectricity passes through it. Traditionally, however, the vapor densityis so high that the electrons cannot be accelerated to a high enoughpotential between scatter events to ionize the inner shell electrons orproduce x-rays from the scattering. In fact, the normal operatingvoltage of flash lamps is only high enough to excite electrons in theouter shells that end up emitting light in the visible, UV, or IRwavelengths. Similarly, neon lights typically have power suppliescapable of 9 kV or more, but due to the high fill pressure only a fewlow energy x-rays are produced. Even the higher voltages are typicallyso low that the few low energy x-rays that may be produced would beabsorbed by the glass envelope. In its simplest form, the flash lamppower supply will consist of a circuit to charge a capacitor thatdischarges when switched on to both trigger and flash the tube.

In continuous operation, a trigger transformer may be used to produce ahigh voltage arc to start the tube, then a lower voltage supply, whichmay be DC, or pulsed DC or AC at a variety of frequencies, will be usedto drive the tube. A neon light will have a ballast and step-uptransformer typically with two secondary windings to generate positiveand negative high voltage. The newer high frequency resonant suppliesfor neon lights convert the line voltage to DC, then produce highfrequency (>20 kHz) AC with a resonant inverter and then use a step-uptransformer. The front end of these power supplies up to the transformeris also very similar to the electronic ballasts used in fluorescentlighting. These tubes are available in many sizes and shapes, which areconceivably adaptable to fluorescent x-ray tube applications.

Some of the above-mentioned systems use pulsed DC supplies that rely oncapacitive discharge. These supplies are frequency limited by the chargeand discharge cycles of the capacitors that also limit the life of thesupply. Many capacitors also discharge slowly compared to potentialspeed of an arc, and so are relatively inefficient at producing x-rays.Resonant supplies are commonly used in fluorescent lighting and resonantsupplies with a high voltage transformer are available for neonlighting. Even the first stages of many high voltage power supplies haveincorporated resonant inverter technology. These high frequency devicescan have smaller and more efficient transformers since they move lesspower per half sine wave, so the overall supply is smaller and moreefficient.

In light of all this, a need exists for a new type of x-ray tube that islower in cost, more efficient, and illuminates a broader area thancurrent technology, while eliminating the troublesome filament. Toachieve these goals it is necessary to integrate new and novelapproaches for increasing the efficiency of x-ray production, and designa new power supply accordingly to create a design for a new class ofx-ray tube and power supply.

Accordingly, the present invention provides a fluorescent x-ray tube andpower supply system that overcomes the problem associated with knownsources of x-rays.

SUMMARY OF INVENTION

The device in accordance with an embodiment of the present inventionconsists of a fluorescent x-ray tube powered by a resonant high voltagepower supply that is suitable for use in an x-ray irradiation device orother device requiring an x-ray source. The fluorescent x-ray tubeconsists of an envelope made of quartz or other suitable non-conductivematerial, with electrodes mounted on opposing sides, and filled with alow-pressure vapor. The high voltage resonant power supply generateshigh frequency alternating current (AC) or direct current (DC) pulses.Arcs are formed between the electrodes when the potential reaches a highenough voltage, usually at or near the power supplies peak voltage. Asthe electrons move through the tube they periodically scatter off vaporatoms or molecules in their paths, ionizing the vapor, and losing someor all of their energy in the process. Scattering and ionization resultin continuum and characteristic x-ray production. Free electrons andions will be accelerated by the potential between the electrodes andperiodically scatter off vapor atoms until they strike an electrode andproduce additional x-rays. The arc in the tube also creates a magneticfield. This field collapses when the arc stops, creating a plasma pinchthat also leads to x-ray production.

Another aspect of the present invention involves the improvement of theefficiency gain in the fluorescent x-ray tube. The efficiency gain inthe fluorescent x-ray tube is a direct result of the excitation of theatoms that comprise the vapor. Once the pressure in the tube is lowenough to sustain a high voltage arc, the mean free path for theelectrons is long enough for the free electrons to gain enough energybetween collisions to produce x-rays when they are scattered. This alsoleads to multiple acceleration zones and therefore multiple x-rayproducing interactions along the length of the arc path. The plasmapinch phenomenon at the end of an arc is also responsible for a greatdeal of the radiation output. In one embodiment of the invention, theefficiency gain is at least five times that of a standard vacuum x-raytube. The fluorescent x-ray tube also operates as a cold cathode deviceusing free electrons from the excited vapor or electrodes thuseliminating the need for the fragile filament.

The operation of the fluorescent x-ray tube is similar in many ways tothe most modern designs for fluorescent lamps, neon lights, or flashlamps, except that the vapor pressure is much lower and the voltage muchhigher. In the operation of these normal everyday lamps, only theoutermost electrons from the vapor atoms are excited, so that theyproduce light mostly in the UV, visible, and infrared regions. Flashx-ray systems are also fundamentally similar since they use vapor arcdischarges to produce x-rays. Flash systems typically have a capacitivedischarge-type supply that is generally suitable to pulsed or lowfrequency, (typically less than 1000 Hz), operation only. To improve theefficiency while reducing size and cost of the power supply, the presentinvention incorporates high frequency resonant inverter technology intothe supply with the addition of a high frequency high voltagetransformer. The inherent difficulty to adapting this technologydirectly to the fluorescent x-ray tubes is designing transformers thatare small enough to operate at high frequency, but big enough toincorporate the insulation needed for the high voltage. In addition,making supplies that can deliver more power is a challenge. In order tomake this x-ray source useful for an irradiation application, it isnecessary to make a supply that is capable of delivering kilowatts ofpower instead of a few hundred watts, and generating voltages of 50 kVor more. To meet these requirements and overcome the problems with knownpower supplies the present invention provides in a new class of x-raysource that can produce x-rays with high efficiency and can be operatedin a continuous fashion.

In an embodiment of the present invention, it is envisioned that manydifferent vapors will be desirable and could be used within the x-raytube to satisfy the need for x-rays of different energies underdifferent circumstances. Each element of the periodic table, whenionized, is capable of giving off different characteristic wavelengthsor energies of photons, including x-rays. In addition, it must be keptin mind that higher energy x-rays have greater penetration power, whichis beneficial for penetrating thicker, higher density, or higher atomicnumber materials. It is known that the output efficiency of a vacuumx-ray tube is proportional to the atomic (Z) number of the anodematerial. Likewise, the efficiency of the fluorescent x-ray tube willalso have a proportional relationship to the atomic number(s) of thevapor constituents in addition to the electrode element(s).

In addition, it will be appreciated by those familiar with x-ray devicesthat the fluorescent x-ray tube may be constructed of various materialsor have various windows installed that are relatively transparent to theradiation energy required by a specific process or application. Theelectrodes may also be composed of materials that are common to the art,and may be selected for their characteristic x-rays, atomic number,melting point, thermal conductivity, electrical conductivity, ionizationpotential, coefficient of expansion, and various other relevantproperties.

The fluorescent x-ray tube of the present invention offers vastlyimproved x-ray production efficiency, a lower production cost for boththe x-ray tube and power supply, less heat generation, and it isdesigned to eliminate the troublesome filament common to existingdesigns. In addition, the present invention easily configured as a broadbeam source, since x-rays are emitted along the entire arc path length.This allows large materials to be placed nearer to the x-ray source,thus minimizing spatial transmission losses in comparison withtraditional point source x-ray tubes. In an alternate embodiment, thefluorescent x-ray tube of the present invention can be collimated and/ordesigned with a short arc gap similar to typical commercially availablearc lamps for imaging applications. The expensive DC power supply usedin traditional x-ray tubes is replaced with a much lower cost resonantsupply, and the x-ray tube itself may be constructed in a much lessexpensive manner than traditional vacuum x-ray tubes, thus making theinvention useful for otherwise cost prohibitive uses.

The fluorescent x-ray tube of an embodiment of the present invention iswell suited to the product irradiation application due to its highefficiency and broad beam capabilities. Material that is to beirradiated can be positioned in close proximity to receive x-rays fromthe tube, in a variety of possible configurations.

In alternate embodiments, the packaging of the irradiation device canalso have several embodiments. In one embodiment, the packaging of thedevice may be a cabinet-type device similar to a microwave oven whereproduct is place inside in order to be treated. In another embodimentthe device is built over or around a material conveyance apparatus forcontinuous or batch treatment much like an airport x-ray scanningsystem. In a third embodiment, it is a flow through device where theproduct, such as liquids or air conveyed materials, flow through an areabeing irradiated. Shielding and safety interlocks are added as needed toprotect operators of the equipment and bystanders.

A fluorescent x-ray tube is beneficial for other typical x-rayapplications as well, including but not limited to x-ray fluorescence,medical and industrial imaging, medical treatment, and x-raylithography.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings. In the drawings,depicted elements are not necessarily drawn to scale and like or similarelements may be designated by the same reference numeral throughout theseveral views.

FIG. 1 shows a basic fluorescent x-ray tube design according to thepresent invention.

FIG. 2 illustrates an alternative x-ray tube design of the presentinvention with electrodes with an angular face.

FIG. 3 shows yet another alternative x-ray tube design according to thepresent invention having bends to angle the electrodes.

FIG. 4 illustrates a multiple electrode x-ray tube according to anaspect of the present invention.

FIG. 5 is a drawing of a gas flow tube design.

FIG. 6 illustrates an alternative drawing of a tube design according tothe present invention with an increased diameter to prevent plating.

FIG. 7 is a drawing of a short arc path tube with an integratedberyllium window according to another aspect of the present invention.

FIG. 8 shows a block diagram of the power supply components according toan embodiment of the present invention.

FIG. 9 is a schematic of a bridge driver for a resonant AC power supplyaccording to the present invention.

FIG. 10 is a schematic of a resonant bridge driver for an AC powersupply according to another aspect of the present invention.

FIG. 11 illustrates an exemplary cabinet for the x-ray source accordingto an embodiment of the present invention.

FIG. 12 illustrates another exemplary cabinet for the x-ray sourceaccording to another embodiment of the present invention.

FIG. 13 illustrates the use of external windings to control arc withinan x-ray tube.

DETAILED DESCRIPTION

According to the present invention, a fluorescent x-ray tube with aresonant power supply is supplied that provides a substantialimprovement over existing x-ray tubes. In the most basic description ofthe operation of an x-ray tube, a pulse of electrons travels through atube, crosses a gap between the electrodes at either end of the arcpath, ionizes the vapor atoms in the path, and creates plasma. Electronsfill the vacant orbitals of vapor atoms and produce photons, includingx-rays characteristic to the vapor atoms. X-rays are also produced whenelectrons that are accelerated by high voltage, scatter off the vaporatoms, which causes them to change directions and emit an x-ray relatedto their change in direction and energy. Additionally, there are ionsbeing accelerated under the same high voltage toward the electrode andthese too can be scattered off other ions, with a resultant productionof x-rays. Some collisions will even occur between ions and electronsaccelerated in opposite directions capable of producing x-rays at twicethe operating potential. Then both electrons and ions strike theelectrodes and lose their energy by both scattering and electronexcitation, leading to further x-ray generation. Finally the magneticfield created by the arc collapses causing a plasma pinch that gives offadditional x-rays.

Vacuum x-ray tubes, flash tubes, and flash x-ray tubes are fundamentallyquite similar. The differences lie principally with the fill gas, fillgas pressure, the operating voltage, and power supply topologies. Vacuumx-ray tubes are usually evacuated to 10⁻⁷ torr or less. These tubes mustbe evacuated to minimize damage to the filament due to ion bombardment.However, due to having a filament, vacuum x-ray tubes do not need vaporfor a supply of free electrons. But, by operating a tube in cold cathodemode, i.e. without a heated filament, and using vapor as a source offree electrons one can eliminate the fragile filament from the x-raysource. Both flash tubes and flash x-ray tubes take advantage of this.

According to the present invention, if a typical flash tube is filled toatmospheric pressure with a vapor and then evacuated, as the vaporpressure in the tube drops to below, for example, 10 millitorr (nitrogencalibrated pressure), the tube will sustain a high voltage potentialacross it, and arcs through the tube produce measurable x-rays. Theexact pressure value depends on fill gas, tube length and other aspectsof tube construction. It is at this point that the vapor density hasdropped low enough (e.g., the mean distance between atoms or moleculesof the vapor is large enough) so that the electrons are capable of beingaccelerated to a high potential between interactions with the vaporatoms, so when they strike the vapor atoms, or the target, x-rays areproduced. If the potential across the tube in kV is greater than thepotential needed to ionize electrons in orbit around the vapor atoms,characteristic x-rays will be emitted when those orbitals are refilledafter ionization.

FIG. 1 illustrates a base fluorescent x-ray tube design that may be usedaccording to the present invention. The basic design of the flash x-raytube includes a quartz envelope 1, two identical electrodes 2 at eitherend of the tube, and contacts 3 for connection to a power supply. In anembodiment, the x-ray tube is a Perkin Elmer ILC Model 8610 flash tubefilled with xenon gas. Initially, the existing flash tube is filled to alower than normal pressure. It was found that a fill pressure range of 4to 7 millitorr (nitrogen calibrated pressure) produced tubes that couldsustain voltages in the 8 kV to 120 kV range, and produce measurablex-rays.

According to the invention, a number of different x-ray tube designs arepossible. FIG. 1 illustrates a slightly modified version of a standardflash tube design. The original design was intended for DC operation.The cathode was conical in shape while the anode was flat, with eachlocated at either end of the quartz envelope 1. There are alsocompositional differences to improve operation; such as the eliminationof the barium complex in the cathode, as well as using a denser tungstenmaterial. The original cathode design would actually entrain the vaporthus reducing the pressure below the useful range over time. For ACoperation, two identical electrodes 2 that have a longer and narrowertip are provided. This gives the tube a more consistent voltageresponse, minimizes mirroring on the envelope from vaporized tungsten,and also minimizes shadowing in the target area by the electrode itself.Still narrower and/or longer electrodes or hollow cathodes may bepreferred for their arc and wear characteristics. It is important tomaximize the space between the electrode tips 2 a where the arcs strikeand the envelope 1 since pinholing of the envelope by the arcs is acommon failure mode. The contacts 3 are typical of flash lamp designs.

FIG. 2 illustrates an alternate x-ray tube design according to thepresent invention. As shown in FIG. 2, the electrodes 4 are cut at anangle so that x-rays originating at the electrode can be directed towardthe material being irradiated giving a small increase in effectiveoutput. This angled cut is common to side window vacuum x-ray tubes. Thetip is slightly rounded to prevent it from having a sharp point adjacentto the envelope, but this design could benefit from having a greaterelectrode to envelope spacing than shown here.

FIG. 3 shows yet another alternate x-ray tube design according to thepresent invention. Instead of cutting the electrode at an angle, FIG. 3shows a design where the tube has a bend 5 so the electrodes 6 face thedirection of the target material. In FIG. 3, only slightly roundedelectrodes 6 are shown that are more typical of flash tube designs, butnot ideal for fluorescent x-ray tubes. A preferred power supply designprovides high frequency alternating current, so both electrodes aretypically identical in shape and material. Other designs intended for DCoperation may have different electrode shapes and/or materialcomposition, depending on whether they are the cathode or the anode,following design strategies that are common to the art of lamp design.The length of the tubes or electrodes may be varied to achieve manydifferent arc lengths. Such variations are common with flash and arclamps, and those designs can readily be adapted to fluorescent x-rayuse.

According to another aspect of the fluorescent x-ray system of thepresent invention, multiple x-ray tubes may be used. Multiple tubearrangements are particularly useful for broad beam irradiationapplications. Such arrangements allow a large area to be irradiated withthe tubes in close proximity with the material, thus minimizing spatialtransmission losses (the R squared losses). In a simple variation,several tubes may be powered in a series or parallel arrangement keepingin mind that twice the voltage is needed when two tubes are in series,and additional or somewhat independent parallel circuitry may be neededin the parallel case to ensure that each tube triggers. Multiplearrangements of such modules may be useful in large area or cabinetirradiation devices. It is also possible to integrate multipleelectrodes into a single vapor filled envelope to accomplish the samething and improve the evenness of the illumination by igniting a largerarea of vapor.

FIG. 4 illustrates an exemplary multiple tube arrangement. A largecircular chamber is provided with a radial arrangement of electrodes 7and contacts 12. The electrodes are held within an insulating material11, and attached to the outer circular electrode 8 by another insulatingpiece 10, which can be all one piece. In an embodiment, the insulatingmaterials (10 and 11) are typically ceramic. However, it should beunderstood that other insulating materials may be used without departingfrom the spirit an scope of the present invention. There is a radiationtransparent window on the opposite side (9) that must be electricallyand mechanically suited to the design in terms of insulationcharacteristics and mechanical strength as required by the significantvacuum in the tube. One can envision numerous other various ways ofarranging the electrodes in a variety of chamber shapes includingspherical. The window material may also be the target, simplifying thedesign somewhat in exchange for the problem of creating a lot of damageto the window during normal use. In alternate embodiments, a linear,radial, or spherical arrangement of electrodes may be designed toproduce ion impacts in a central region at effectively twice the appliedvoltage. These broad area irradiation designs can easily be ten timesmore efficient geometrically at delivering an x-ray dose over a widearea than a point x-ray source. By also considering the typical 5 to 10time improvement this invention offers, it is possible to achieve 100times more efficient use of power over traditional x-ray sources in someapplications.

FIG. 5 illustrates the use of continuous vapor flow or periodic vaporinjection in an x-ray tube. Although this aspect of the presentinvention is illustrated using a single x-ray tube, these same conceptsmay be extended to multiple tube arrangements. Gas-puff devices are welldocumented, but would be difficult to implement at high frequency,although a pulsed device may be preferred in some instances formaintaining proper pressure within the tube. Vapor flow is attractivefor its added cooling; its ability to carry away vaporized ions from theenvelope or electrodes; and its ability to regulate the tube pressureexternally. In its simplest form the envelope will have tubes 13attached behind both electrodes 14, so one can function as an inlet andthe other an outlet. This can improve the longevity of the tube and makeit more viable for high output or continuous use applications. It isimportant to note that the inlet and outlet gas flow connections need tobe electrically isolated to prevent arcing as is well-known in the art.It may also be advantageous to have holes drilled through the electrodesto provide for the vapor flow and at the same time make it a hollowcathode design as discussed below.

Since one of the principle failure mechanisms for a cold cathode tube isdue to plating of the electrode material along the tube walls, and ionimpacts are the most significant cause of the electrode vaporization, itis possible to extend tube life by controlling the location of theplating so that it does not degrade the x-ray transmission or provide aconductive path along the inside of the envelope. It is possible toreduce the plating along the main body by several techniques. FIG. 6illustrates one technique for reducing plating according to the presentinvention. As shown in FIG. 6, the tube diameter 19 is increased in theregion just inside the electrodes 20 causing most of the plating tooccur in the adjacent area. The arcs occasionally bounce off theenvelope at various points along the length of the arc path so a largerdiameter is a means to distribute these events over a larger surfacearea and thus prolong tube life. Increasing the volume can also allow anincrease in power by increasing the number of possible ionization eventsand/or the magnitude of the plasma pinch. One may consider deflectingmost of the ions away from the electrodes by designing in a radius thatis to tight for them to travel in or to use electrostatic or otherdeflection devices, but these solutions are not very practical due tothe fact that most of the ions come from a region within severalmillimeters of the electrode.

Just as small arc length arc lamps can be used in focused lightingapplications such as spotlights, a small arc length fluorescent x-raytube may be used according to the present invention for focused x-rayapplications such as medical imaging and therapy, industrialradiography, and x-ray lithography applications. FIG. 7 illustrates theuse of small length are lamps in an embodiment of the fluorescent x-raytube of the present invention. Electrodes 25 are located closer together(approximately a 1 mm gap). The envelope 26 has been enlarged in thevicinity of the arc as is typical with an arc lamp. While the lamp canbe made with or without a more x-ray transparent window, an embodimentuses drawing illustrates a beryllium window 27 attached to the envelope26 using a design that is typical of a side window vacuum x-ray tube. Awindow may be desired when low x-ray energies are needed, in particularbelow 20 keV. An x-ray window that is relatively transparent to thedesired energy may be installed in the tube's envelope. Such windows aretypically constructed of thin aluminum, beryllium foil, glass quartz, orother similarly low atomic number material. Window assemblies may alsoinclude a ring that may be used for mounting, grounding, and/orcollimation.

In the realm of plasma physics, magnetic confinement has beenestablished as a principal method for containing and controlling plasma.The fluorescent x-ray tube is no different. According to one aspect ofthe present invention, by placing inductive windings around the tube, anincrease in the current in the pulse is achieved. Since the arcs arepreceded by a buildup of free charges at the electrodes, the inductanceof the windings resists the current flow and allows for greater chargebuildup and hence higher current when the pulse does occur. Theinductor(s) may be passive, having only a fixed resistance, or active,each with its own internal current flow. In its simplest form, one longinductor may extend over the length of the tube with adequate spacing ormaterial composition to be relatively transparent to x-rays. In apreferred embodiment, an inductor is located near the electrodes at eachend. Other electrostatic and/or magnetic field generating devices may beused around the tube for the purpose of confining and controlling thearcs. By keeping the arc centered in the tube, potential damage to theenvelope is minimized. Additionally, having windings around the tube canprovide a means for triggering the tube, or controlling the triggeringvoltage.

FIG. 13 illustrates the use of external windings 80 to control the arcwithin the tube. According to Lenz's Law, the electromagnetic field inthe tube will produce a current in the windings 80 that produces a fieldback on the tube 79 working against the motion of the tube current. Thismay lead to greater charge buildup in the plasma adjacent to theelectrode, so when it does arc, the arc contains more charge. Thecircuit can be an active or passive design with the simplest being shownwith a resistor 77 and a capacitor, or other voltage source or storagedevice 78 in the circuit. If the tube is driven in AC, the R-L-C circuitcan be designed to resonate at the same frequency. The electrostaticfield from the windings also helps keep the arc centered in the tube inthe region within the windings.

According an embodiment of the invention using magnetic confinement, thefrequency of the power supply can be adjusted such that it coincideswith the arc timing and transformer resonance characteristics creating aresonant state within the tube and power supply. For example, the L8610tube sustains an arc with a minimum duration of approximately 200nanoseconds, so a power supply frequency in the 2 to 5 megahertz rangewould be required. The duration is largely a function of arc pathlength, and can vary from a few nanoseconds to a few milliseconds, and aresonant frequency can in theory be found over the entire range, given asuitable transformer and switching power supply. For this reason, thepresent invention may extend to higher frequencies when the arcdurations are shorter, limited only by the ability to construct suitablehigh frequency, high voltage transformers. It is also possible toenhance the resonant effect and create high-pressure nodes periodicallyalong the length of the tube where x-ray production would be quite highby tuning the Crooks bands spacing with the voltage and pressure suchthat the arc length is an integer multiple of the individual Crooks bandspacing. The addition of electrostatic and/or magnetic confinement canfurther increase the intensity of interactions in the nodes by confiningthem to a smaller region of the tube.

In yet another embodiment of the invention, cooling of the tubes may beaccomplished through convective or forced air cooling, or static orcirculated liquid cooling. One of the most attractive options, and apredominant method used with high power x-ray tubes, is the use of astatic or oil filled container that provides both cooling and electricalinsulation. FIG. 11 illustrates an exemplary system with tubes 70 placedin oil-filled trays (65 and 71). FIG. 12 illustrates an alternate systemthat further includes a heat exchanger 76 and an oil pump 75 for runningat higher power. Evaporative cooling techniques can be used as well andare particularly suitable for high power applications. For high-poweredin-line systems a large heat exchanger may be incorporated with acirculated coolant design that can even be located outside a building tominimize heat buildup inside a structure.

Another aspect of the invention involves the selection of vapor usedwithin the x-ray tube. The selection of the vapor relates to theparticular application. In one example, for a fluorescent x-ray tubedesigned for irradiating meat up to 10-12 cm thick, 30 keV x-rays areattractive since as much as 80% of the x-ray flux hitting the meat willbe absorbed. Accordingly, for this application, xenon, which has acharacteristic K x-ray emission at about 30 keV, would be selected asthe fill gas for the fluorescent x-ray tube. Several other gases areattractive for other applications. For instance, krypton would beuseful, with its 13 keV x-ray emissions, for the irradiation of thinnerand/or lower atomic number materials. A heavier vapor such as mercury(70 and 80 keV) would be suitable for thicker and/or higher atomicnumber or more dense materials such as steel. In each of the abovecases, the operating voltage of the power supply must be adjustedaccordingly. In general, the higher the atomic weight of the vapor thehigher the characteristic x-ray energy, and the higher energy conversionefficiency. Other atoms present in other gases or gas mixtures used inlighting systems known to the art such as halogens, sodium, or variousmetal halides, would be suitable as well for special applications. Anyelement, or combination of elements, that form a suitable vapor may beused to obtain specific characteristic x-ray emission energies. Anymixtures of the above gases may also be suitable in order to change theenergy spectrum. An additional quench gas such as noble gases or methanemay be needed in some mixtures.

By designing the irradiation system of the present invention to produceradiation at several different energies it is possible to get betterdose uniformity throughout the target material, particularly when it isthicker or higher in density. The irradiation system could use one tubefilled with the required mixture of gases; or several tubes, each with aprincipally mono-species gas fill, could be used together as anirradiation package. In addition, the fill gas in the tubes may bedesigned to produce desired x-ray emissions of the K, L, M, or Ntransitions of certain fill gas elements that may useful separately, orin combinations.

In addition, in order to achieve x-ray production in a vapor filledtube, the vapor pressure must be very low, generally in the range of0.01 to 100 millitorr depending on the desired voltage, fill gas, andtube construction. In this pressure range as the pressure is decreasedthe breakdown voltage of the vapor in the tube increases. Depending ontube construction, it will take from a few kV to a hundreds of kV totrigger the x-ray tube and give off x-rays. This method does howeverextend into the gamma ray range of energies as it is possible to maketubes with pressures in 10⁻⁶ to 10⁻⁴ torr range that require MeV energypower supplies.

In designing the present invention, the tube pressure and voltage shouldbe matched, since if there is too little voltage for a given pressurethere will only be a faint glow discharge across the tube. The glowdischarge regime is a very inefficient and low power regime with regardto x-ray production. We have measured it and found it to be 25 to 100times worse than a fluorescent x-ray tube designed in accordance withthis invention or 5-10 times worse than even a traditional vacuum x-raytube. If the pressure is too high for a given voltage, the tube will arctoo soon and the x-ray energy will be lower than desired. While the arcsmay be longer in duration, there will not be a very efficient conversionof energy into x-rays. In order to create x-rays efficiently, the pulsemust be very fast, typically much less than a microsecond, so it isimportant for the voltage to be just high enough so an arc initializes,but current limited so that the vapor arc discharge is not sustained forvery long. Experiments have shown that once the initial arc isestablished, which takes from 10's to 100's of nanoseconds depending ontube length, sustaining the arc leads to decreased x-ray yields. At theextreme limit, the discharges fall under the class of dischargephenomena called pseudosparks in which charges build up at one electrodeuntil it becomes unstable and then arc across the tube, but there isinsufficient current flow through the circuit to sustain the arc.Pseudosparks are also known as “hollow cathode discharge” since arcformation is enhanced by the presence of relatively sharp edges on theelectrode. The simplest version of a “hollow cathode design is a hollowcylinder, but it may also be a large area plate where arcs form at theedges. The plate area may include holes in it to promote arc and thusx-ray development over a large surface area. In principle though, sincepseudosparks are initiated in response to a free charge buildup near theelectrode, designing an electrode with more surface area is beneficial.The present invention further contemplates to increasing the chargeavailable for the arc by constructing an electrode in the form ofmultiple concentric cylinders. In alternate embodiments, the chargeavailable may be increased by increasing the diameter or elongating theelectrode.

Further, very short arcs are the most efficient mechanism for producingx-rays. In the absence of an electrode redesign or an externalelectromagnetic field generating device, the best way then to push morepower through the tube is to increase the frequency, hence thedevelopment of a high frequency supply.

The present invention further contemplates the use of a high voltageresonant power supply to produce the vacuum arcs needed to drive thex-ray tube. The basic block diagram of a resonant AC power supply isshown in FIG. 8a. The direct current power module at the front end ofthe power supply may contain the power factor correction circuit withthe primary voltage supply and also an auto ranging feature that permitsoperation at multiple common voltages and frequencies. For example, inone embodiment, the auto ranging feature would permit operation at110V/60 Hz and 220V/50 Hz. They can be incorporated together into aninput module 28. This primary voltage supply can be fixed or adjustablefrom a few volts to five kilovolts or more, and may be a battery, alinear supply or use buck, boost, or other common voltage conversiontopologies . The direct current power module may also include a currentcontrol circuit. It will be used to drive the power supply's highfrequency switching controller 29 and resonant power module 30 that makeup the AC inverter. The high frequency resonant controller may in theoryoperate at a frequency from a few Hz to 100 MHz or more, but thepreferred embodiment is in the 2 kHz to 10 MHz range due to the overallefficiency, resonant characteristics, and transformer operatingfrequencies. It is also useful in many cases to operate at frequenciesabove 20 kHz so as not to be in the audible range. The high frequencyswitching controller is also a possible location for both voltage andfrequency control circuits either instead of or in addition to similartypes of controls at the direct current power module. A transformer ortransformers 31 are used to raise the voltage needed to power the x-raytube 32. The higher the input voltage to the bridge, the lower thewinding ratio in the transformer(s), and the better its performance andfrequency range will be.

FIG. 9 shows an exemplary simple circuit design for a high frequencyswitching controller or bridge driver 29 based on a Texas Instruments(formerly Unitrode) UC 3875 controller 39. Numerous other controllersand a variety of driver circuit designs are commonly available fordriving resonant power supplies, and suitable versus may be adapted foruse with the present invention.

FIG. 10 is a more detailed diagram of the bridge driver 29 according toan aspect of the present invention. The resonant bridge circuit (50-58)includes transformers (55), ballasts (56), and a tube (58). The outputof the H bridge is connected to two transformers 55. The primarywindings are in anti-phase, while the secondary windings are in series.This way the circuit, and more to the point, the high-voltage insulationhas to be designed for only half of the required high voltage. Thisexample inverter is a zero voltage switching resonant bridge. It isshown here with isolation transformers for driving the MOSFET's, butthey may also be driven with optical coupling devices. A Royer powersupply is another attractive topology commonly used in cold cathodeplasma applications such as plasma displays, and it is also possible tobase a design on a half bridge instead of a full or H bridge. It is alsopossible to use the push pull or other converter topology, but as withthe half bridge they can only achieve half the voltage of the fullbridge given the same level of voltage rating of the components.

A single transformer or two transformers with the primaries in phase maybe used, but it would then have to be designed for twice the voltage,increasing both its size, expense, and design difficulty. Alternatively,a single transformer with two secondaries wound in opposite directionsand wired in series may be used, but this design requires a core designthat is not commonly available in an appropriate material for therequired frequency range. It is also possible to have the primary andsecondary windings on different legs of the core to make the highvoltage insulation design easier, or they may be on the same leg toachieve better efficiency. In one specific embodiment the high voltagehigh frequency transformer design incorporates tubular insulatorsbetween winding layers made of Teflon, Kapton or other similarly goodinsulator to achieve insulation between layers rated anywhere from 20 to250 kV DC allowing AC operation in the hundreds of kV. When properlydesigned, the transformers 55 act as the ballast in the circuit. It ispossible to add other ballast components between the transformer(s) 55and the tube as shown, but additional ballast components to diminish thex-ray intensity, and therefore may also be deleted.

Presenting the material to be irradiated to the x-ray source can beaccomplished in many ways traditional to industries such as x-rayfluorescence (XRF) analysis. It may be a closed cabinet device wherematerial is placed in a cabinet that is then closed, and then safetyinterlocks are actuated to allow the irradiation process to beinitiated. One example of a closed cabinet device could be very similarto a microwave oven both in terms of construction, safety interlocks,and controls, with one principle exception being the use of appropriateshielding for x-rays. X-ray tubes may be oriented above, below, to thesides, or in any combination that is suitable to and helps achieveuniform irradiation of the material.

FIGS. 11 and 12 illustrate exemplary closed-cabinet devices. Theseclosed-cabinet devices could incorporate traditional microwave ovenconstruction features such as safety interlocks and controls (68 and72). In addition, these closed-cabinet devices would require additionalshielding 74. As illustrated, an exemplary embodiment may use sixfluorescent x-ray tubes 70 with two sets of three located in oil filedtrays (65 and 71) above and below the sample chamber. A resonant powersupply 66 supplies power. Each tube is in series with two high-voltagetransformers/ballasts 67. The device may further include amicroprocessor control board 68 and control interface such as a touchscreen 72. A shielded door 69 opens and closes for easy access to theshielded chamber. The chamber is shielded on all sides with appropriatematerial such as lead. A cooling system may also be incorporatedincluding cooling fans 73 that can either blow air across the tray 71 oracross a heat exchanger 76. When using a heat exchanger 76, the heatexchanger 76 could be convective or use pumps 75 to circulate the oil.

Another embodiment of the present invention takes advantage of the smallgains in performance that can be obtained by using reflected x-rayenergy. X-rays are not efficiently scattered off of most materials sothat the “reflected” energy is typically a factor of hundred times lessthan the incidence energy. There are some techniques that improve theefficiency, such as using materials that are easily excited by theincident X-rays and then fluoresce their own characteristic x-rays at aslightly lower energy. Such a material is functionally like a secondarytarget in XRF. In the example where xenon is the vapor used in the tube,a material such as tin (Sn) may be used as a secondary target/shield.The performance gain is small, but may be useful in some circumstances.It is also possible to enhance the reflectance by using low atomicnumber material such as hydrogenated material, since it is a superiorx-ray scatterer to metals. It should be noted, however, that x-raysbreak the bonds in polymers, causing them to degrade over time.Therefore it is advisable to encapsulate such material. A thirdtechnique is to use materials that efficiently diffract the importantx-ray energies; these types of materials are relatively expensivehowever and will likely increase the overall cost.

By using the fact that the “reflected” energy is low and it falls offwith the square of the distance, it is also possible to produce x-raydevices that are open ended. One such familiar device is the airportluggage scanner. These types of configurations can be used for manyin-line irradiation applications. And, for even more powerful x-raysystems, it is usually only necessary to force the x-rays to “reflect”off more surfaces before exiting the chamber to have it reach safelevels. This makes the material path a little more convoluted, but it isstill practical in many cases. With liquid samples it is possible tohave the fluid (such as water or juice) flow right past an appropriatelyinsulated tube within piping, as is the case with many UV sterilizationproducts.

A fluorescent x-ray tube is beneficial for applications in the x-rayfluorescence (XRF) industry as well. One significant problem in XRF isthat the somewhat parallel beam from a typical x-ray tube will scatteroff a sample being analyzed in such a way that the intensities at agiven energy may be more due to surface features such as striations anddiffraction phenomena rather than composition. The more randomized andbroader beam inherent to some fluorescent x-ray tube designs canminimize this problem. Lower cost is also a major factor in making thistechnology useful in a broader class of applications.

By designing the tube with a short arc path, or with collimation,fluorescent x-ray tubes may also be used for industrial or radiographicimaging applications. The present invention may also be adapted totherapeutic applications as well. One example of which is a smalldiameter x-ray needle incorporating this technology that could beinserted into the body for the purpose of destroying cancerous tumors.

Due to the efficiency improvements offered by a plasma x-ray source,these types of sources have been studied extensively in relation tox-ray lithography applications where traditional x-ray tubes aregenerally not powerful enough, particularly when x-ray optical elementsare incorporated in the design. The fluorescent x-ray tube of thepresent invention is also attractive for this application and may beconfigured as a broad beam source for simple contact masking techniquesor as a smaller point source if the angular distribution from the sourceneeds to be restricted. Various optical elements such as diffractive,multiplayer, or capillary optics could be used with this style of tubein the same fashion as they are used with other x-ray sources.

Fluorescent x-ray tubes may be incorporated into a device for thepurpose or irradiating materials for the purpose of killingmicroorganisms or pests. The device may be used for the irradiation ofmaterials such as food, water, and other beverages, and medicalequipment or waste. It may be constructed in the form of a closedcabinet device, or an in-line device over or around a conveyance orwithin a liquid stream. The fluorescent x-ray tube is also suitable forother applications including but not limited to x-ray lithography, x-rayfluorescence, medical and industrial imaging and medical therapeuticdevices.

Further, although the previously described high voltage, high frequencyresonant power supply has been described in the context of x-rayproduction, the power supply according to the present invention may haveother applications. For example, it is contemplated that the powersupply is of equal value in other vacuum arc discharge applications. Thepresent embodiment allows a better way of driving the followingapplications while still being able to operate as pseudo-DC powersupply. With appropriate control features the power supply of thepresent invention can generate single pulses and mimic the DC supplywith a HV switch or the DC supply with a capacitive discharge.

In one embodiment of the invention, the power supply can be used toproduce vacuum arc discharges in vacuum arc deposition equipmentdesigned to produce coatings. This equipment currently has similarlimitations to other equipment previously mentioned in that theygenerally use a high voltage DC source and a capacitive discharge orother pulse forming system, and they have a pre-ionizing device toassist in triggering the arcs. The high voltage resonant power supply islower in cost, more efficient and self trigger by over voltage. Inaddition, with a high frequency device, each arc can have much lesscharge, or in other words fewer ions, so it is possible to create muchthinner and uniform coatings. While at higher power that may be achievedwith this power supply coatings can be produced much quicker or thickerthan with lower frequency supplies.

In another embodiment it is envisioned that the power supply could beused in vacuum metal refining. Vacuum metal refining is used to improveuniformity and reduce grain size in alloys that otherwise would haveoverly large substructures. It is also used for degassing metals. Theprocess works in much the same way as vacuum arc deposition in thatmaterial is vaporized and one location and electrically deposited byvacuum arc in another, but in vacuum metal refining the metal isdeposited in a mold where an ingot is formed.

In still another embodiment it is envisioned that the power supply canbe used in ion implantation devices. As mentioned previously, one of thetechnical challenges in designing a fluorescent x-ray tube wasovercoming the tendency for the fill gas to be implanted into theelectrode creating a reduction in vacuum. This property can be used toadvantage in cases where ion implantation is used to change theproperties of a material for applications such as wafer fabrication forsolid-state devices. The basic operation and advantages are similar tothe two previous types of equipment, but ion implantation requires evenhigher voltages, an area where the power supply designed for thisinvention would work exceptionally well.

Although the high voltage, high frequency resonant power supply for theproduction of x-rays has been described in the above-referenceapplications, it should be understood that the power supply describedherein may be used in other applications without departing from thespirit and scope of the present invention.

While the invention may be adaptable to various modifications andalternative forms, specific embodiments have been shown by way ofexample and described herein. However, it should be understood that theinvention is not intended to be limited to the particular disclosedembodiments. Rather, the invention is to cover all modifications,equivalents, and alternatives falling within the spirit and scope of theinvention as defined by the claims. Moreover, the different aspects ofthe disclosed system and methods may be utilized in various combinationsand/or independently.

What is claimed is:
 1. An apparatus for producing an x-ray comprising: aresonant plasma x-ray tube comprising an envelope, two electrodes, and alow-pressure vapor within the x-ray tube; and a resonant high voltagepower supply operatively coupled to the x-ray tube and operable toproduce high-frequency AC to form arcs between the electrodes.
 2. Theapparatus of claim 1 wherein the x-ray tube has a short arc path for thepurpose of producing a focused source for radiographic applications. 3.The apparatus of claim 2 wherein the arc path is 0.1 to 3 mm.
 4. Theapparatus of claim 1 wherein the x-ray tube has a larger diameter in thevicinity of the arcs.
 5. The apparatus of claim 1 having a plurality ofelectrodes.
 6. The apparatus of claim 5 wherein the plurality ofelectrodes are arranged radially.
 7. The apparatus of claim 5 whereinthe plurality of electrodes are arranged spherically.
 8. The apparatusof claim 1 wherein said electrodes have an angular face so that theelectrodes face toward a target.
 9. The apparatus of claim 1 whereinsaid x-ray tube has bends to angle said electrodes toward a target. 10.The apparatus of claim 1 wherein the vapor is xenon.
 11. The apparatusof claim 1 wherein the vapor is krypton.
 12. The apparatus of claim 1wherein the vapor is mercury.
 13. The apparatus of claim 1 wherein thevapor is a mixture of gases.
 14. The apparatus of claim 13 wherein themixture of gases includes a quench gas.
 15. The apparatus of claim 1wherein the working vapor pressure in the x-ray tube is a pressurebetween 0.01 to 100 millitorr.
 16. The apparatus of claim 1 furthercomprising a vapor inlet and a vapor outlet coupled to the x-ray tube.17. The apparatus of claim 1 further comprising an x-ray transparentwindow integrated with the x-ray tube.
 18. The apparatus of claim 1further comprising an inductive device around the x-ray tube operable tocontrol the characteristics of the arc.
 19. The apparatus of claim 18wherein the inductive device comprises a magnetic field producingdevice.
 20. The apparatus of claim 18 wherein the inductive devicecomprises an electrostatic field producing device.
 21. The apparatus ofclaim 1 further comprising x-ray optics.
 22. The apparatus of claim 1further comprising means for cooling the x-ray tube.
 23. The apparatusof claim 1 wherein the resonant high voltage power supply operates in afrequency range of 2 kHz to 10 MHz.
 24. The apparatus of claim 1 whereinthe resonant high voltage power supply is adapted to be controlled by afeedback loop based on a current measurement.
 25. The apparatus of claim1 wherein the resonant high voltage power supply is adapted to becontrolled by a feedback loop based on an x-ray intensity measurement.26. The apparatus of claim 1 wherein the resonant high voltage powersupply is adapted to be microprocessor controlled.
 27. The apparatus ofclaim 1 further comprising a reflective material.
 28. The apparatus ofclaim 27 wherein the reflective material is a low atomic numberscattering material.
 29. The apparatus of claim 27 wherein thereflective material is tin.
 30. A system for irradiating an object withbroad beam x-rays comprising: a resonant plasma x-ray tube comprising anenvelope, two electrodes, and a low-pressure vapor within the x-raytube; a resonant high voltage power supply operatively coupled to thex-ray tube and operable to form arcs between the electrodes by producinghigh-frequency AC; and a cabinet for enclosing the x-ray tube.
 31. Thesystem of claim 30 wherein said cabinet comprises a fan for circulatingair to cool the x-ray tube.
 32. The system of claim 30 wherein saidcabinet comprises tubing for circulating liquid through the cabinet tocool the x-ray tube.
 33. The system of claim 30 wherein the cabinet isshielded.
 34. The system of claim 30 wherein the cabinet is aninterlocked cabinet enclosure.
 35. The system of claim 30 comprising aplurality of x-ray tubes enclosed in the cabinet.
 36. The system ofclaim 35 wherein said plurality of x-ray tubes are connected inparallel.
 37. The system of claim 35 wherein said plurality of x-raytubes are connected in series.
 38. The system of claim 35, furthercomprising a means for exchanging the gas in order to produce radiationat selective energies.
 39. The system of claim 30 wherein the cabinetcomprises controls for controlling power of the resonant high voltagepower supply.
 40. The system of claim 30 wherein the cabinet comprisescontrols for controlling the frequency of the resonant high voltagepower supply.
 41. The system of claim 30 wherein said cabinet comprisesan oil-filled container for cooling of the x-ray tube.
 42. A system forirradiating an object with x-rays comprising: a resonant plasma x-raytube comprising an envelope, two electrodes, and a low-pressure vaporwithin the x-ray tube; a resonant high voltage power supply operativelycoupled to the x-ray tube and operable to form arcs between theelectrodes by producing high-frequency AC; and an open-ended enclosurefor enclosing and shielding the x-ray tube.
 43. The system of claim 42further comprising a fan for circulating air to cool the x-ray tube. 44.The system of claim 42 further comprising tubes for circulating liquidto cool the x-ray tube.
 45. The system of claim 42 further comprisingcontrols for controlling the conveyance system.
 46. The system of claim42 further comprising controls for controlling the frequency of theresonant high voltage power supply.
 47. The system of claim 42 furthercomprising controls for controlling power of the resonant high voltagepower supply.
 48. The system for irradiating an object of claim 42further comprising a conveyance system for continually moving theobjects through open-ended enclosure.
 49. The system of claim 42comprising a plurality of x-ray tubes.
 50. The system of claim 49wherein said plurality of x-ray tubes are connected in parallel.
 51. Thesystem of claim 49 wherein said plurality of x-ray tubes are connectedin series.
 52. The system of claim 49, further comprising a means forexchanging the gas in order to produce radiation at selective energies.53. A method of generating x-rays for irradiating an object comprising:providing one or more resonant plasma x-ray tubes having a low-pressurevapor within the x-ray tubes; providing a resonant high-voltage powersupply; generating a resonant high voltage power output includinghigh-frequency AC; and applying the resonant high voltage power outputto the x-ray tube in order to produce arcs across the electrodes andproduce x-rays.
 54. The method of claim 53 comprising providing amixture of vapors in order to produce x-rays at multiple energies. 55.The method of claim 53 further comprising cooling the one or more x-raytubes.
 56. The method of claim 55 wherein the cooling is performed usingair.
 57. The method of claim 55 wherein the cooling is performed bycirculating liquid.
 58. The method of claim 53 further comprisingconveying the object through the generated x-rays.
 59. The method ofclaim 53 further comprising continuously flowing the vapor in the one ormore x-ray tubes.
 60. The method of claim 53 further comprisingperiodically injecting the vapor into the one or more x-ray tubes. 61.The method of claim 53 further comprising applying an electrostaticfield around the one or more x-ray tubes thereby controlling the arcwithin the x-ray tubes.
 62. The method of claim 53 further comprisingapplying a magnetic field around the one or more x-ray tubes therebycontrolling the arc within the x-ray tubes.
 63. An apparatus forgenerating x-rays for irradiating an object comprising: one or moreresonant plasma x-ray tubes having a low-pressure vapor within the x-raytubes; means for providing a resonant high voltage power signalincluding high-frequency AC by adjusting the frequency of thehigh-voltage power supply to coincide with the arc timing in the one ormore x-ray tubes; and means for applying the resonant high voltage powersignal to the x-ray tube in order to produce arcs across the electrodesand produce x-rays.
 64. The apparatus of claim 63 further comprising ameans for conveying the object through the generated x-rays.
 65. Theapparatus of claim 63 further comprising means for cooling the one ormore x-ray tubes.
 66. The apparatus of claim 63 further comprising meansfor applying a magnetic field around the one or more x-ray tubes therebycontrolling the arc within the x-ray tubes.
 67. The apparatus of claim63 further comprising means for applying an electrostatic field aroundthe one or more x-ray tubes thereby controlling the arc within the x-raytubes.
 68. The apparatus of claim 63 further comprising means forperiodically injecting the vapor into the one or inure x-ray tubes. 69.The apparatus of claim 63 further comprising means for continuouslyflowing the vapor in the one or more x-ray tubes.